Field of the Disclosure
The present disclosure relates to frequency conversion of laser radiation by means of non-linear interaction of laser radiation with a suitable non-linear crystal. In particular, the disclosure relates to an improved design of the external cavity for frequency conversion in which the optical power density inside the volume and at faces of the crystal is reduced so as to substantially increase the crystal's lifetime. Furthermore, the disclosure relates to the improved manufacturability of the external cavity for frequency conversion built with a standard, single set of optical components which can be used for a wide range of input powers.
Prior Art Discussion
As known, there are wavelengths that cannot be directly accessed with modern laser technology. Nonlinear frequency conversion techniques allow generating laser radiation at these wavelengths in the UV, visible and IR spectral ranges.
Conceptually, the term frequency conversion includes generation of second, third, fourth and higher order harmonics, sum frequency generation and other nonlinear processes leading to a change of frequency of laser light. Typically, the frequency conversion of a continuous wave (“CW”) laser radiation requires a resonator with an internally placed nonlinear crystal due to a relatively low single pass conversion efficiency of the internal generation of the higher order harmonic generation by the nonlinear crystal. The most widely used configurations include intracavity and external cavity resonators provided with nonlinear crystals. This disclosure relates to the external cavity approach.
Referring to FIG. 1, a ring 4-mirror bowtie frequency converter 10, representative of a great variety of external ring cavity configurations, is frequently used due to its versatility and relative ease of implementation. The resonator 10 is configured with a non-linear (“NL”) crystal 12 located in a beam waist between two concave mirrors M3, M4. The placement of crystal 12 in the beam's waist provides high frequency conversion efficiency. However, the crystal in this position suffers from high optical power density of the frequency-converted beam at the crystal's output face and inside the crystal's volume. As a result, during the conversion, high energy photons at the converted wavelength are incident on the output surface of crystal 12 and can degrade this surface, while a high power density around the beam waist can damage the bulk of crystal 12.
The damage to the output surface can be minimized, of course, by reducing the power density on the output surface via reducing the size of the waist. As the latter reduces, the beam divergence increases reducing optical power density on the output surface of the crystal. However, scaling down the beam waist increases an already high power density within the waist in the crystal which can initiate damage in the bulk of the crystal.
A few solutions improving the useful life of a NL crystal have been suggested. The most common solution has been to use a crystal with a large cross section and translate the crystal in a pre-defined pattern to expose a fresh area when one area is damaged. The crystal is replaced after there is no useable area left. Although this technique increases the useful lifetime of a single crystal, it does not address the fundamental aspects of the crystal damage and involves a bulky, complicated and costly mechanical motion mechanism.
A need therefore exists for a frequency converter resonator designed so that a NL crystal has a long useful life.
Besides the crystal useful life, the frequency conversion efficiency of the nonlinear crystal within the external converter resonator is another concern. The conversion efficiency critically depends on making the resonator impedance matched, among other factors. The impedance matching involves the optimization of cavity parameters (most commonly transmission of the input mirror) to maximize the coupling of light into the resonant cavity. Maximum coupling is obtained when the round trip losses of the converter (including frequency conversion) are equal to the transmission T of the coupling mirror M1. Referring again to FIG. 1, the resonator is impedance matched when the coefficient of transmission T of the mirror M1 substantially matches the single pass frequency conversion efficiency in crystal 12, provided other cavity losses are negligible.
It is impossible to achieve impedance matching of the converter resonator for different input powers using the same set of physical components, such as an input collimator, mirrors and crystal. As the input light power changes, it is necessary to change an NL crystal's length, and/or transmission of coupling mirror M1 and/or diameter of the input beam in order to keep the cavity impedance matched. Any of these modifications is not a simple operation and requires time and effort which increase the cost of manufacturing frequency converters.
A need, therefore, exists for a frequency convertor resonator designed with a set of elements easily adaptable to satisfy the impedance matching condition of the resonator in response to a wide range of input light powers without replacing any of the physical elements of the resonator.